A laying vessel is a floating unit comprising a production line for producing the underwater pipeline, and equipped at the stern with a lay ramp, which forms an extension of the production line and provides for laying the underwater pipeline on the sea bed. Though the following description refers specifically to laying pipelines in the sea, the present invention also applies to any stretch of water large and deep enough to accommodate a laying vessel.
At present, underwater pipelines are produced and laid using two main methods, depending on the depth of the sea bed.
A first method is to produce the pipeline on a vertical production line, and lay the pipeline vertically, so that the portion of the pipeline extending between the vessel and the sea bed assumes a “J” shape. This method is particularly suitable for laying deep-sea underwater pipelines. A second method is to produce the pipeline on a substantially horizontal production line, and lay the pipeline using a lay ramp, which, in the work configuration, guides and supports the pipeline along an arc-shaped path having a first portion above sea level, and a second portion below sea level. Using this method, the pipeline being laid assumes an “S” shape between the vessel and sea bed, and various types of vessels can be employed, such as pontoons, barges, semisubmersible and single-hull vessels. Pontoons and barges are the underwater-pipeline laying vessels most commonly used in the past; semisubmersible vessels are particularly suitable for laying underwater pipelines in bad weather (rough sea); and single-hull vessels have the advantage of being fast, have ample pipe storage space, and can accommodate relatively long production lines with a high output rate.
Laying underwater pipelines using the second method calls for relatively good weather conditions, so much so that, in rough-sea conditions, laying is suspended: the pipeline being laid is left on the sea bed, possibly attached to a salvage cable; and the lay ramp is set as high as possible above sea level in a so-called “survival configuration”.
Moreover, the lay ramp must be movable into various work configurations, depending on the depth of the sea bed and the characteristics of the pipeline, and into an in-transit configuration allowing unobstructed movement of the vessel.
Accordingly, the lay ramp comprises an actuating device for altering its configuration and its position with respect to the vessel.
One particular type of lay ramp, designed to assume various configurations and positions, is described in the Applicant's U.S. Pat. No. 4,257,718, which describes a semisubmersible underwater-pipeline laying vessel equipped with a lay ramp comprising an inside ramp hinged to the vessel; an outside ramp hinged to the inside ramp; and an actuating device comprising a first actuating assembly for rotating the inside ramp with respect to the vessel, and a second actuating assembly for rotating the outside ramp with respect to the inside ramp. The first actuating assembly is connected to the inside ramp, and is movable along a substantially vertical runner fixed to the vessel; and the second actuating assembly is connected to the outside ramp, and is movable along a horizontal runner fixed to the vessel.
In the technical solution described in U.S. Pat. No. 4,257,718, the horizontal runner comprises rails fixed to the ceiling of the deck.
Though highly efficient, this technical solution has the drawback of being specially designed for a semisubmersible vessel, and not easily adaptable to a single-hull vessel, the advantages of which—longer production line, higher output, more pipe storage space, faster travelling speed—make it preferable to a semisubmersible vessel in many applications.
A single-hull vessel, having no stern top deck to which to fix the horizontal rails, cannot be equipped with a lay ramp of the type described in U.S. Pat. No. 4,257,718, but has a well at the stern at least partly housing the lay ramp, so lay ramps comprising an inside ramp and an outside ramp have cable actuating assemblies. Cables, however, are not an ideal solution, by requiring guide pulleys, and by only providing for one-sided restraint. That is, a cable only provides for restraint when pulled, performs no restraining function when compressed or bent, and so fails to provide for stable configurations and positions of the lay ramp.